There is a lot of duplication in the pmu_*_el0_disabled helpers,
and as we're going to modify them shortly, let's move all the
common stuff in a single function.

No functional change.
Reviewed-by: NChristoffer Dall <cdall@linaro.org>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NChristoffer Dall <cdall@linaro.org>